The separation of mixtures containing multiple components which, because of solubility, density or some other characteristic can separate into distinct phases, is a process of importance to industry. Such two phase mixtures can result from a number of processes.
In the production of crude oil, water/oil emulsions are produced. These two phase liquid-liquid mixtures can comprise simply water and oil or surfactants (both naturally occurring as well as deliberately added) can be present. The two phase mixture must be separated to recover the valuable oil product.
Dewaxing of lube oils and other petroleum streams can be accomplished by crystallization of the wax with or without a solvent. This results in formation of a two phase liquid(oil)-solid(wax) mixture which must be separated to recover the oil and wax products. Even after this primary two phase mixture separation, additional dispersed two phase mixtures can result from subsequent processing of each of the initially separated phases. For example U.S. Pat. No. 2,232,722 is directed at a dewaxing process in which the single phase dewaxed oil solution, resulting from a prior two phase wax/oil separation, is chilled to promote phase separation Additional solvent may be added to the solution to facilitate the phase separation.
A traditional method for separation of solid-liquid mixtures and for separation of two phase immiscible liquid-liquid mixtures is by gravity or "gravity-enhanced" (e.g. centrifuges) settling. A typical settler is an enlarged vessel with or without baffles which creates the settling zone, thus providing the residence time required for the gravity induced separation of a two phase system. The heavier liquid or solid phase settles to the bottom and the lighter phase rises to the top but within the settler there is an intermediate zone at which there is still present a dispersed mixture of the two phases. The process of settling may be carried out batchwise or continuously.
Settlers can provide excellent separation of two phase mixtures with near complete recovery of each phase in a high state of purity. However, limitations occur with certain two phase systems when only very small density differences exist between the two phases and/or there is stabilization of the dispersed phase. Such systems require very long settling times and can result in prohibitively large settling vessels for continuous operation One example of such a hard-to-settle two phase system would be a solids in liquid suspension in which the particle size of the solids is very small and/or the density of the solids is near that of the liquid as might occur in a petroleum dewaxing process. Another example would be a finely dispersed, surfactant stabilized liquid-liquid micro-emulsion, as occur in water/oil emulsions in which surfactants are present, such as are encountered in crude oil production.
Another example of two phase mixtures which require separation occurs in the production of lube oil or other petrochemical feedstocks, by liquid-liquid extraction processing. Liquid-liquid extraction in either a batchwise or continuous co-current or countercurrent mode is used to separate petroleum molecules, for example, to separate undesired aromatic compounds from distillates to increase lube oil quality and to separate highly valued molecules such as benzene, toluene, xylene for chemicals production. In the liquid-liquid extraction process, the feed stream is contacted with a suitable solvent, for example, phenol, furfural, acetone or N-methyl pyrrolidone, resulting in the formation of a dispersed two phase liquid-liquid mixture, these being the raffinate solution and the extract solution, within the extraction tower. The raffinate solution and extract solution are withdrawn separately from the extraction tower as the main products but within the tower there exists an intermediate zone functioning as a settling zone in which is present a dispersed mixture of extract solution/raffinate solution.
Previously, efforts have been directed at the subsequent processing of the extract and raffinate solutions from the extraction tower to separate and recover the solvent, as well as to further purify the lube oil base, which typically comprises the raffinate. These subsequent processing steps also encounter dispersed two phase mixture which must be separated.
U.S. Pat. Nos. 3,725,257 and 3,985,644 are both directed at the extractive separation of an aromatic-rich stream from a petroleum fraction. The extract solution from the extraction tower is cooled which results in the formation of an aromatic-rich hydrocarbon phase and a solvent-rich phase. The solvent-rich phase is recovered and recycled either to an intermediate point or to the top of the extractor while the hydrocarbon-rich phase is passed to a distillation zone. This process is deficient in that the solvent returned to the extractor contains significant quantities of aromatic hydrocarbons. Thus, recycling this recovered solvent to the top of the extractor will require significantly more solvent for effective removal of aromatics from the aromatic-containing feedstock.
U.S. Pat. No. 2,754,249 also discloses the extraction of a hydrocarbon fraction to remove non-paraffinic compounds. The extract is de-oiled by the addition of an anti-solvent capable of reducing the solubility of the extracted hydrocarbons in the solvent. This results in the formation of a dispersed two phase mixture which must be separated so that the hydrocarbon phase can be recovered and the phase containing solvent and anti-solvent passed on to a distillation zone to recover the solvent and anti-solvent streams. The solvent, substantially devoid of anti-solvent, then is mixed with solvent recovered from the raffinate phase and recycled to the extraction system. This process depends upon the use of large amounts of anti-solvent to effect the required separation. Thus, the solvent-rich phase must be distilled to remove large quantities of anti-solvent, resulting in this process being energy intensive.
An alternative technology to settling, particularly useful for hard-to-settle two phase systems, is membrane separation. Microfiltration or ultrafiltration membranes can effectively separate two phase solid-liquid and immiscible liquid-liquid mixtures. The membrane provides a physical barrier to passage of the dispersed liquid or solid phase while the continuous liquid phase is permeated through the membrane under a pressure gradient.
Such a membrane process can recover some portion of the continuous phase with very high purity but is generally limited to relatively low less than 50%, recovery levels. As a result the dispersed phase, the retentate, from the membrane process will still contain large amounts of the continuous phase.
In practice, membrane and settling technology can be and occasionally are used together in a combination process. Membrane processing can be used to concentrate a two phase mixture to a small volume retentate which then requires a relatively smaller settler for subsequent treatment.
The sequence can also be reversed
U.S. Pat. No. 4,432,866 is directed to a method for separating a mixture into a first fraction and a second fraction. The method comprises passing the mixture into a settling/decantation zone where the solution is separated into a first fraction relatively rich in a first component and a second fraction relatively deficient on the first component. One of the separated fractions subsequently is passed to a membrane separation zone for further processing, and separation to produce a retentate stream further enriched in one or the other component (as the case may be) and a permeate further depleted in that component. The permeate or retentate, as appropriate is recycled to the initial processes (i.e. dewaxing, extraction etc.) which produced the initial mixture separated by the process. An anti-solvent can be added to the initial mixture to facilitate separation in the decantation/settling zone. In addition to or in place of the anti-solvent, the mixture can be chilled to induce phase separation.
GB 1,456,304 teaches a process for treating waste aqueous streams. A waste water stream is sent to a surge tank wherein solids are separated out of the stream. A fraction of the waste water is withdrawn from the surge tank and sent to an ultrafiltration unit wherein an oil fraction is recovered and a water fraction is recovered. In one embodiment a residual emulsion further separated from the recovered oil phase is cycled back to the ultrafiltration unit for additional processing.
"Oily Bilge Water Treatment With a Tubular Ultrafiltration System" Harris et al. Journal of Engineering for Industry, November 1976 pg 1215-1220 describes a system wherein oily bilge water is sent to a settler and the bottoms phase (the water phase) is sent to an ultrafiltration unit to further separate residual oil from the water, thereby producing a retentate containing the residual oil and a permeate constituting oil free water. From the diagram in the article it is seen that the permeate and retentate either combined or separately could be returned to the top of the settling tank.